BBC - GCSE Bitesize: Radioactive atoms Radioactive atoms There are three ways an unstable nucleus can decay. It may give out: Two protons and two neutrons – the same as a helium nucleus Beta particle High energy electromagnetic radiation Ionisation ability Ionising radiation is radiation that has enough energy to cause other atoms to lose electrons and form ions. The different forms of radiation have different levels of ionisation abilities: Alpha particles – very ionising Beta particles – moderately ionising Gamma rays – weakly ionising You need to know the effect of alpha and beta decay on the nucleus. The nucleus of an atom can be represented as: Where: A = atomic mass (number of protons + neutrons) Z = atomic number (number of protons) X = chemical symbol (as shown on the periodic table) Alpha decay When an alpha particle is emitted from a nucleus the nucleus loses two protons and two neutrons. This means the atomic mass number decreases by 4 and the atomic number decreases by 2. A new element is formed that is two places lower in the Periodic Table than the original element. Example: Radon decays into polonium when it emits an alpha particle. Here is the equation for that radioactive decay.

Beta decay In Beta decay a neutron changes into a proton plus an electron. The proton stays in the nucleus and the electron leaves the atom with high energy, and we call it a beta particle. When a beta particle is emitted from the nucleus the nucleus has one more proton and one less neutron. This means the atomic mass number remains unchanged and the atomic number increases by 1. Example: Carbon-14 is a radioactive isotope of carbon. (It's a carbon atom with 8 neutrons instead of the usual 6.) Here is the equation for the beta decay of carbon-14 into nitrogen. Page: Radioactivity Radioactivity Radioactivity refers to the particles which are emitted from nuclei as a result of nuclear instability. Because the nucleus experiences the intense conflict between the two strongest forces in nature, it should not be surprising that there are many nuclear isotopes which are unstable and emit some kind of radiation. The most common types of radiation are called alpha , beta , and gamma radiation, but there are several other varieties of radioactive decay. Radioactive decay rates are normally stated in terms of their half-lives , and the half-life of a given nuclear species is related to its radiation risk .

The different types of radioactivity lead to different decay paths which transmute the nuclei into other chemical elements. Examining the amounts of the decay products makes possible radioactive dating . Radiation from nuclear sources is distributed equally in all directions, obeying the inverse square law . Go Back Alpha Radioactivity Composed of two protons and two neutrons, the alpha particle is a nucleus of the element helium. Because of its very large mass (more than 7000 times the mass of the beta particle) and its charge, it has a very short range . It is not suitable for radiation therapy since its range is less than a tenth of a millimeter inside the body. Its main radiation hazard comes when it is ingested into the body; it has great destructive power within its short range. In contact with fast-growing membranes and living cells, it is positioned for maximum damage. Alpha particle emission is modeled as a barrier penetration process. The alpha particle is the nucleus of the helium atom and is the nucleus of highest stability . Go Back Alpha Barrier Penetration The energy of emitted alpha particles was a mystery to early investigators because it was evident that they did not have enough energy, according to classical physics, to escape the nucleus.

Once an approximate size of the nucleus was obtained by Rutherford scattering, one could calculate the height of the Coulomb barrier at the radius of the nucleus. It was evident that this energy was several times higher than the observed alpha particle energies. There was also an incredible range of half lives for the alpha particle which could not be explained by anything in classical physics. The resolution of this dilemma came with the realization that there was a finite probability that the alpha particle could penetrate the wall by quantum mechanical tunneling . Using tunneling, Gamow was able to calculate a dependence for the half-life as a function of alpha particle energy which was in agreement with experimental observations. Alpha Beta Gamma Radiation Alpha Beta Gamma Radiation Alpha Particles- An alpha particle has two protons and two neutrons, so it has a positive charge. (Since it has two protons it is a helium nucleus.) It is produced from large nuclei. When an atom emits an alpha particle, the atom's mass number decreases by four due to the loss of the four nucleons in the alpha particle. The atomic number of the atom goes down by exactly two, as a result of the loss of two protons – the atom becomes a new element.

Examples of this are when uranium becomes thorium, or radium becomes radon gas due to alpha decay. Alpha particles are commonly emitted by all of the larger radioactive nuclei such as uranium, thorium, actinium, and radium, as well as the transuranic elements. Unlike other types of decay, alpha decay as a process must have a minimum-size atomic nucleus which can support it. The smallest nuclei which have to date been found to be capable of alpha emission are the lightest nuclides of tellurium (element 52), with mass numbers between 106 and 110. The process of emitting an alpha sometimes leaves the nucleus in an excited state, with the emission of a gamma ray removing the excess energy. Beta Particles- Beta emission is when a high speed electron (negative charge) leaves the nucleus. Beta emission occurs in elements with more neutrons than protons, so a neutron splits into a proton and an electron. The proton stays in the nucleus and the electron is emitted. Gamma Rays- A nucleus which is in an excited state may emit one or more photons (packets of electromagnetic radiation) of discrete energies.

The emission of gamma rays does not alter the number of protons or neutrons in the nucleus but instead has the effect of moving the nucleus from a higher to a lower energy state (unstable to stable). Gamma ray emission frequently follows beta decay, alpha decay, and other nuclear decay processes. Positron- Positron emission involves a particle that has the same mass as an electron but a positive charge. The particle is released from the nucleus when a proton splits into a neutron and positive electron. Separation of some radioactive emissions Note the path of the beta particle is curved more than the alpha. This is due to the mass. A wiffle ball can curve more than a more massive baseball. Alpha, Beta and Gamma Radioactivity Alpha, Beta and Gamma Radioactivity Alpha Decay An alpha particle [xxi] (a) is a helium nucleus, i.e. does not contain electrons, it is composed of two neutrons and two protons, thus it has a positive two charge. An alpha particle does not have the ability to penetrate through most substances; a piece of paper will stop most alpha particles. This type radiation is very dangerous [xxii] if inhaled or ingested, in general it will not escape the human body.

One of the home concerns is the release of Radon gas. Radon is an alpha emitter and can be inhaled since it is a gas. The Radon gas while in your lungs may decay and thus emit an alpha particle that the soft tissue of your lungs will absorb the energy, which could lead to lung cancer. If you are a smoker than the chances of the decay of the radon gas occurring inside your lungs is much greater. Some of the radon gas atoms will stick to the inside of your lungs due the tar from smoking. If the radon sticks to your lung the chance of the radioactive decay happening while inside your body is much greater because non-smokers exhale most of the radon on each breath. By losing two protons we have change the chemical makeup of the atom, since the number of protons control what element you have. The alpha particle contains a large amount of energy because of the mass present, thus the kinetic energy of the particle is transferred to what ever it strikes including cells in the human body. It is an ionizing radiation since it has a positive two charge, it can stripe electrons away from neutral atoms. In addition to the release of the alpha particle normally gamma rays are also released.

Beta Decay A beta particle [xxiii] (b) is an electron emitted from the nucleus of an atom. This is not one of the orbiting electrons, which is very important. The process is the changing of a neutron into a proton. The -1 on the beta particle is not a negative number of protons; it is just used to balance the equation on charge. The last symbol represents an anti-neutrino. A neutrino is a chargeless, massless particle that travels at the speed of light but is not electromagnetic. It is required to balance the equation and beyond the scope of this discussion. There are also positive beta particles (positrons). Since a beta particle [xxiv] is much less massive than an alpha particle it can penetrate through a greater thickness in materials, while it is still relatively easy to stop. Since the number of protons changes during beta decay the element, which released the beta particle, will change during the reaction. Note the atomic mass did not change. Gamma Decay A gamma ray (s)[xxv] (g) is emitted during either alpha or beta decay. These rays are electromagnetic radiation and have properties similar to light.

Since they are electromagnetic radiation they travel at the speed of light and the gamma photons are massless. A gamma ray [xxvi] is a way for the nucleus to lose energy and move into a more stable state, just as electrons want to be in the most stable orbitals, the nucleus also wants to be in the lowest energy state. There is no change in the substance by the emission of a gamma ray. Gamma rays are extremely penetrable and it requires several inches of lead to stop them. Note gamma rays were shown in the decays of both alpha and beta particles. Note: The video files listed below are the same video information in two different file formats. The Windows Media Video (.wmv) is in two different size formats, the 720 x 480 size will require a broadband connection, but has higher resolution and more depth. The 320 x 240 wmv file is a smaller file size and can be loaded onto a Microsoft Zune. Video Files: Atomic, Nuclear and Modern Physics - Alpha, Beta and Gamma Processes What is the Alpha Particle? Rutherford Elements and Atoms: Chapter 19 What is the Alpha Particle?

Rutherford No one contributed more than Ernest Rutherford (1871-1937; see photo in Chemical Achievers at Chemical Heritage Foundation) to an understanding of radioactivity and its domain, the atomic nucleus, in the early years of the 20th century. Working in J. J. Thomson's laboratory, Rutherford distinguished between two different forms of radioactivity, alpha (α) and beta (β) [ Rutherford 1899 ]. He worked out the time-dependence of radioactive decay and introduced the term half-life [ Rutherford 1900 ]. With Frederick Soddy, he realized that radioactive decay actually tranforms an atom of one element into an atom of a different element [ Rutherford & Soddy 1902 ]. [1] This work was sufficient to earn Rutherford a Nobel Prize in 1908. [2] But some of his most important work was still ahead. Large-angle scattering of α particles was first reported in his laboratory [ Geiger & Marsden 1909 ].

His correct interpretation of that scattering led to the realization that most of the mass of an atom is concentrated in a tiny core or nucleus [ Rutherford 1911 ]; thus it is to Rutherford that we owe the nuclear atom and nuclear physics. The experiments which found a physically measurable quantity associated with atomic number were also carried out in his laboratory [ Moseley 1913, 1914 ]. During the World War, Rutherford discovered that some atoms could be induced to fall apart in a process of artificial transmutation [ Rutherford 1919 ; view photo of apparatus at Science Museum, London]. Rutherford characterized the α particle in work extending over several years with a variety of co-workers. The selection reproduced below represents the final step in the identification of the α particle as a positively-charged helium atom. (I would say "helium nucleus," but Rutherford had not yet discovered the nucleus.) In importance to the development of understanding about the atom, this paper does not rank with those cited above. It does, however, illustrate the simple and careful experimental methodology of Rutherford.

The Nature of the α Particle from Radioactive Substances [3] Ernest Rutherford & Thomas Royds, Philosophical Magazine 17, 281-286 (1909) The experimental evidence collected during the last few years has strongly supported the view that the α particle [4] is a charged helium atom, but it has been found exceedingly difficult to give a decisive proof of the relation. In recent papers, Rutherford and Geiger [5] have supplied still further evidence of the correctness of this point of view. The number of α particles from one gram of radium have been counted, and the charge carried by each determined. The values of several radioactive quantities, calculated on the assumption that the α particle is a helium atom carrying two unit charges, have been shown to be in good agreement with the experimental numbers. In particular, the good agreement between the calculated rate of production of helium by radium and the rate experimentally determined by Sir James Dewar [6] , is strong evidence in favour of the identity of the α particle with the helium atom.

[7] The methods of attack on this problem have been largely indirect, involving considerations of the charge carried by the helium atom and the value of e/m of the α particle. The proof of the identity of the α particle with the helium atom is incomplete until it can be shown that the α particles, accumulated quite independently of the matter from which they are expelled, consist of helium. For example, it might be argued that the appearance of helium in the radium emanation was a result of the expulsion of the α particle, in the same way that the appearance of radium A is a consequence of the expulsion of an α particle from the emanation. If one atom of helium appeared for each α particle expelled, calculation and experiment might still agree, and yet the α particle itself might be an atom of hydrogen or of some other substance. [8] We have recently made experiments to test whether helium appears in a vessel into which the α particles have been fired, the active matter itself being enclosed in a vessel sufficiently thin to allow the α particles to escape, but impervious to the passage of helium or other radioactive products. [9] The experimental arrangement is clearly seen in the figure.

The equilibrium quantity of emanation from about 140 milligrams of radium was purified and compressed by means of a mercury-column into a fine glass tube A about 1.5 cms. long. This fine tube, which was sealed on a larger capillary tube B, was sufficiently thin to allow the α particles from the emanation and its products to escape, but sufficiently strong to withstand atmospheric pressure. After some trials, Mr. Baumbach succeeded in blowing such fine tubes very uniform in thickness. The thickness of the wall of the tube employed in most of the experiments was less than 1/100 mm., and was equivalent in stopping power of the α particle to about 2 cms. of air. Since the ranges of the α particles from the emanation and its products radium A and radium C [10] are 4.3, 4.8, and 7 cms. respectively, it is seen that the great majority [11] of the α particles expelled by the active matter escape through the walls of the tube. The ranges of the α particles after passing through the glass were determined with the aid of a zinc-sulphide screen.

[12] Immediately after the introduction of the emanation the phosphorescence showed brilliantly when the screen was close to the tube, but practically disappeared at a distance of 5 cms. Such a result is to be expected. The phosphorescence initially observed was due mainly to the α particles of the emanation and its product radium A (period 3 mins.). In the course of time the amount of radium C, initially zero, gradually increased, and the α radiations from it of range 7 cms. were able to cause phosphorescence at a greater distance. The glass tube A was surrounded by a cylindrical glass tube T, 7.5 cms. long and 1.5 cms. diameter, by means of a ground-glass joint C. A small vacuum-tube V was attached to the upper end of T. The outer glass tube T was exhausted by a pump through the stopcock D, and the exhaustion completed with the aid of the charcoal tube F cooled by liquid air. By means of a mercury column H attached to a reservoir, mercury was forced into the tube T until it reached the bottom of the tube A.

[13] Part of the α particles which escaped through the walls of the fine tube were stopped by the outer glass tube and part by the mercury surface. If the α particle is a helium atom, helium should gradually diffuse from the glass and mercury into the exhausted space, and its presence could then be detected spectroscopically by raising the mercury and compressing the gases into the vacuum-tube. In order to avoid any possible contamination of the apparatus with helium, freshly distilled mercury and entirely new glass apparatus were used. Before introducing the emanation into A, the absence of helium was confirmed experimentally. At intervals after the introduction of the emanation the mercury was raised, and the gases in the outer tube spectroscopically examined. After 24 hours no trace of the helium yellow line was seen; after 2 days the helium yellow was faintly visible; after 4 days the helium yellow and green lines were bright; and after 6 days all the stronger lines of the helium spectrum were observed. The absence of the neon spectrum shows that the helium present was not due to a leakage of air into the apparatus. [14] There is, however, one possible source of error in this experiment.

The helium may not be due to the α particles themselves, but may have diffused from the emanation through the thin walls of the glass tube. In order to test this point the emanation was completely pumped out of A, and after some hours a quantity of helium, about 10 times the previous volume of the emanation, was compressed into the same tube A. The outer tube T and the vacuum-tube were removed and a fresh apparatus substituted. Observations to detect helium in the tube T were made at intervals, in the same way as before, but no trace of the helium spectrum was observed over a period of eight days. [15] The helium in the tube A was then pumped out and a fresh supply of emanation substituted. Results similar to the first experiment were observed. The helium yellow and green lines showed brightly after four days. [16] These experiments thus show conclusively that the helium could not have diffused through the glass walls, but must have been derived from the α particles which were fired through them. In other words, the experiments give a decisive proof that the α particle after losing its charge is an atom of helium. Other Experiments.

We have seen that in the experiments above described helium was not observed in the outer tube in sufficient quantity to show the characteristic yellow line until two days had elapsed. Now the equilibrium amount of emanation from 100 milligrams of radium should produce helium at the rate of about .03 c.mm. per day. The amount produced in one day, if present in the outer tube, should produce a bright spectrum of helium under the experimental conditions. It thus appeared probable that the helium fired into the glass must escape very slowly into the exhausted space, for if the helium escaped at once, the presence of helium should have detected a few hours after the introduction of the emanation. [17] In order to examine this point more closely the experiments were repeated, with the addition that a cylinder of thin sheet lead of sufficient thickness to stop the α particles was placed over the fine emanation tube. Preliminary experiments, in the manner described later, showed that the lead-foil did not initially contain a detectable amount of helium.

Twenty-four hours after the introduction into the tube A of about the same amount of emanation as before, the yellow and green lines of helium in this case after one day was of about the same intensity as that after the fourth day in the experiments without the lead screen. It was thus clear that the lead-foil gave up the helium fired into it far more readily than the glass. In order to form an idea of the rapidity of escape of the helium from the lead some further experiments were made. [18] The outer cylinder T was removed and a small cylinder of lead-foil placed round the thin emanation-tube surrounded the air at atmospheric pressure. After exposure for a definite time to the emanation, the lead screen was removed and tested for helium as follows. The lead-foil was placed in a glass tube between two stopcocks. In order to avoid a possible release of the helium present in the lead by pumping out the air, the air was displaced by a current of pure electrolytic oxygen [19] . The stopcocks were closed and the tube attached to a subsidiary apparatus similar to that employed for testing for the presence of neon and helium in the gases produced by the action of the radium emanation on water (Phil.

Mag. Nov. 1908). The oxygen was absorbed by charcoal and the tube then heated beyond the melting-point of lead to allow the helium to escape. The presence of helium was then spectroscopically looked for in the usual way. Using this method, it was found possible to detect the presence of helium in the lead which had been exposed for only four hours to the α rays from the emanation. After an exposure of 24 hours the helium yellow and green lines came out brightly. These experiments were repeated several times with similar results. A number of blank experiments were made, using samples of the lead-foil which had not been exposed to the α rays, but in no case was any helium detected. In a similar way, the presence of helium was detected in a cylinder of tinfoil exposed for a few hours over the emanation-tube. [20] These experiments show that the helium does not escape at once from the lead, but there is on the average a period of retardation of several hours and possible longer.

The detection of helium in the lead and tin foil, as well as in the glass, removes a possible objection that the helium might have been in some way present in the glass initially, and was liberated as a consequence of its bombardment by the α particles. The use of such thin glass tubes containing emanation affords a simple and convenient method of examining the effect on substances of an intense α radiation quite independently of the radioactive material contained in the tube. We can conclude with certainty from these experiments that the α particle after losing its charge is a helium atom. Other evidence indicates that the charge is twice the unit charge carried by the hydrogen atom set free in the electrolysis of water. [21] University of Manchester, Nov. 13, 1908 Notes [1] The idea of one element changing into another was foreign to the chemistry and physics of the time; it was associated with fantasies of a pre-scientific era. Rutherford is supposed to have urged his colleague, "Don't call it transmutation.

They'll have our heads off as alchemists." [Pais 1986] [2] It is ironic that a man who quipped, "All science is either physics or stamp collecting," [Blackett 1963] won a Nobel Prize in chemistry. Because his work touched on the nature of the elements, chemistry was the field in which it was recognized with a Nobel. [3] Communicated by the Authors. [original note --CJG] [4] Fairly early in the study of radioactivity, three different types of radioactive phenomena were identified. Rutherford first distinguished between two kinds which he unimaginatively labeled α and β [ Rutherford 1899 ]. (Following that precedent, the third kind of radioactivity was called γ.) Rutherford found that one kind of radiation (β) was more penetrating than the other (α), and thus distinguished them on the basis of their range. But what were these rays? Were they particles like cathode rays? Were they energy like light or X-rays? β rays were identified as electrons by the early 20th century. γ rays are a high-energy form of electromagnetic radiation like visible light or X-rays but with a much shorter wavelength.

[5] Proc. Roy. Soc. A. lxxxi, pp. 141-173 (1908). [original note --CJG] [6] Proc. Roy. Soc. A. lxxxi. p. 280 (1908). [original note --CJG] [7] How might one go about characterizing the α particle? The first step was to distinguish it from other radioactive phenomena, which Rutherford first did by noting that it was not as penetrating as β particles. In fact, Rutherford went on to measure how far α particles from various radioactive materials could travel through air. (See note 12 below.) Then measuring the charge to mass ratio (e/m) and, if possible, the charge separately, would be helpful. After all, the e/m ratio proved to be crucial to characterizing cathode rays ( chapter 16 ), and that line of inquiry developed several techniques for measuring e/m and the charge. With this information in hand, Rutherford could tell that the particle in question was not something as light as an electron or as heavy as an atom of uranium, but was more like a helium atom. Was it a helium atom?

The next step would be to try to collect α particles and see if the collected material is in fact helium. Indeed, William Ramsay and Soddy observed production of helium in the presence of an α-emitting substance, namely radium, as early as 1903 [ Ramsay & Soddy 1903 ]. [8] What Rutherford calls radium emanation is now known to be a particular isotope of radon gas, namely 222Rn. And radium A is actually an isotope of polonium, 218Po. (See next chapter for more on isotopes.) Rutherford notes that just because helium is produced at the same rate as α particles does not mean that α particles are helium. He gives two examples of radioactive decay processes which produce α particles: radium --> "radium emanation" + α and "radium emanation" --> radium A + α. In the first case, radium emanation is produced at the same rate as the α particle; it is not an α particle, however, but another fragment (by far the larger, in fact) of the decaying radium atom. [9] Wait a minute!

How can the vessel be thin enough to let α particles through but not helium atoms if α particles are charged helium atoms? This is not as contradictory an apparatus as it may appear. The α particles, whatever they are, are ejected from the decaying atom with great speed, enabling them to penetrate thin walls that are impervious to ordinary gases including helium. The idea here is to make a barrier that will let through only α particles, and to examine those accumulated α particles once they have slowed down. And we will see that Rutherford & Royds make sure that α particles can get through ( note 12 ) and that ordinary helium cannot ( note 15 ). [10] Radium C is actually an isotope of polonium, 214Po. [11] The α particles fired at a very oblique angle to the tube would be stopped in the glass. The fraction stopped in this way would be small under the experimental conditions. [original note --CJG] [12] How might one measure the range of α particles? All one needed was a way to detect the particles. Then simply vary the distance between the source of the particles (i.e., the radioactive substance) and the detector.

There were already several ways of detecting particles emitted by radioactive materials. We have already seen the use of photographic plates (by Becquerel, chapter 17 ) and a device sensitive to electric charge (by Curie, chapter 18 ). Rutherford used a technique based on the fact that α particles could make certain phosphorescent materials, such as zinc sulfide, visibly sparkle. One could observe and manually count the number of sparkles (or scintillations) one saw (in a dark room, of course). About this time, Hans Geiger and Rutherford invented an electrical device to detect and amplify the charge α particles carried [Rutherford & Geiger 1908a]. At any rate, scintillations on the zinc sulfide screen outside the thin tube show that α particles can get out of that tube. [13] The apparatus surrounds the thin tube containing the emanation with empty space, pumped free of any impurity. This was the space in which the α particles were to be collected. [14] Most important, the space surrounding the thin tube was to be free of any extraneous helium before the start of the experiment.

So Rutherford & Royds look for helium in that space by looking for the lines characteristic of the spectrum of helium. The spectrum is a particularly sensitive method of identifying helium; that is, helium can be detected in very small quantities. At first they see no helium, but after the experiment has been running for a few days, they see some. Conclusion: helium enters the collection space during the course of the experiment. While they see helium, they do not see neon. Conclusion: the helium they see did not leak in from the atmosphere, for if it did, neon would leak in with it. [15] How do Rutherford & Royds make sure that helium cannot get through the thin-walled tube? They put helium in the tube, then look for it in the outer collection tube and find none. [16] For good measure, they repeat the first experiment. Thus they know that helium is collected reproducibly in the outer tube and only under conditions in which α particles leave the thin-walled tube. [17] At this point, the main question of the experiment has been answered conclusively, as stated at the end of the previous paragraph.

The experimenters are now pursuing a loose end, a slightly unexpected observation: helium does not accumulate in the outer tube as rapidly as α particles are produced and leave the inner tube. Why not? Maybe it takes a long time for them to wiggle free from the glass walls of the outer tube where many of them are stopped. How might one test this explanation? Try stopping the α particles with a different material, and see if the accumulation of helium is faster or slower. [18] At this point, the attention of the investigators appears to have shifted to the ability of lead to retain captured α particles. In fact they are investigating one more possible explanation of helium in their earlier experiments. "How do we know," they ask, "that the helium was not originally in the glass wall of the outer tube and was only released by exposure to α particles?" [19] That the air was completely displaced was shown by the absence of neon in the final spectrum. [original note --CJG] [20] They eliminated the possibility that α-particle bombardment released helium already present in the glass in two ways. First, they changed the outer glass tube for one made of lead foil and later tinfoil, and still got helium.

Second, they showed that they obtained helium only after bombardment by α particles. In effect, they could collect α particles within metal foils, remove the source of the particles, and then release helium from the foils: that helium came from the already-collected α particles. [21] None of the experiments described in this paper demonstrates that the α particle is a charged particle. Other experiments carried out previously showed that it carries two units of positive charge [Rutherford & Geiger 1908b]. References P. M. S. Blackett, "Memories of Rutherford" in J. B.

Birks, ed., Rutherford at Manchester Proceedings of the Royal Society (London) A 82, 495-500 (1909) 26, 1024-1034 (1913) and 27, 703-713 (1914) Abraham Pais, Proceedings of the Royal Society (London) 72, 204-207 (1903) Philosophical Magazine 49, 1-14 (1900) Philosophical Magazine 37, 581-587 (1919) Ernest Rutherford & Hans Geiger, "An Electrical Method of Counting the Number of α-Particles from Radio-Active Substances," Proceedings of the Royal Society (London) A 81, 141-161 (1908a) Ernest Rutherford & Hans Geiger, "The Charge and Nature of the α-Particle ," Proceedings of the Royal Society (London) A 81, 162-173 (1908b) Radioactivity - ANSTO Radioactivity Useful

 links What is radioactivity?

What is radioactivity? What is radioactive decay? Are there different types of radiation and how far can radiation penetrate? Find answers to your questions here. Shortcuts: Radioactivity and radiation 'Radioactivity' is the energy and mass released by spontaneous changes in the nucleus of an atom. 'Radiation' is energy that travels as waves or particles. Heat, light, sound, microwaves, radar, radio waves, X-rays, alpha and beta particles, and gamma rays are all forms of radiation. Radioactive decay How Radiation Travels Radioactive decay is the spontaneous radioactive disintegration of an atomic nucleus, resulting in the release of energy. Some atoms are stable while others are unstable and decay, emitting radiation to achieve a stable state. The emissions from an unstable atom's nucleus, as it decays, can be in the form of alpha, beta or gamma radiation. When an atom decays, it changes into another isotope, or form, of the same element or into a completely different element, in a process called transmutation. Different isotopes of the same element differ in the number of neutrons in their nuclei. Some elements reach stability via a series of steps through several isotopes, or 'daughter products'.

One example is uranium-238 (U-238), which, through the process of radioactive decay, will eventually become a stable isotope of lead. However, this process takes billions of years. Along the way, as the U-238 isotope's initial energy declines, it will transmute via a series of elements, each more stable than the last - thorium, radium, radon, polonium and bismuth - before it stabilises as lead. Alpha decay In alpha decay, a positively-charged particle is emitted from the nucleus of an atom. This alpha particle consists of two protons and two neutrons (the same structure as a helium-4 nucleus). Although alpha particles are normally highly energetic, they travel only a few centimetres in air and are stopped by a sheet of paper or the outer layer of dead skin. Beta decay In beta decay, a particle is emitted from the nucleus of an atom. This beta particle is an electron with either negative or positive electric charge. Beta particles may travel metres in air and several millimetres into the human body. Most beta particles may be stopped by a small thickness of a light material such as aluminium or plastic.

Gamma decay Gamma decay occurs because the nucleus of an atom is at too high an energy state. The nucleus 'falls down' to a lower energy state, emitting a high energy photon known as a gamma particle in the process. Gamma particles travel in a wave-like pattern at the speed of light. They can only be stopped by a dense material such as lead, steel, concrete or several metres of water. Types of radiation There are two main types of radiation which includes non-ionising and ionising radiation. 'Non-ionising radiation' has less energy than ionising radiation but can still excite molecules and atoms causing them to vibrate faster. Near ultraviolet, visible light, infrared, microwave, radio waves, and low-frequency RF (longwave) are all examples of non-ionising radiation. 'Ionising radiation' has enough energy to change the chemical composition of matter by forcing an atom or molecule to give up an electron, therefore 'ionising' it. These electrically-charged particles are called ions. Ionising radiation sources include alpha and beta particles, gamma rays, neutrons and cosmic rays. More about ionising radiation There are three types of ionising radiation: alpha, beta and gamma.

When ionising radiation hits any material or the human body, that material or body does not itself become radioactive, but the different particles will have different levels of penetration and effects. Sources of ionising radiation include neutrons and cosmic rays. Neutron radiation occurs within a nuclear reactor. Cosmic radiation originates from stars in outer space, making up part of the 'background radiation' that we are all exposed to every day. Penetration of radiation The discovery of radioactivity has delivered many benefits, but it must be handled with care. A sheet of paper, or even the skin of our bodies, will stop alpha particles, while a thin sheet of perspex or glass will stop beta radiation. However, the energy of both can cause damage to cells if they enter the body through inhalation, swallowing or wounds. Thick barriers of lead, water and concrete are necessary to stop much more penetrating and damaging gamma radiation. Penetration of radiation- a guide Fission versus fusion When atoms split apart or join together, energy is released. Splitting atoms is called nuclear 'fission', while joining atoms is called nuclear 'fusion'. Nuclear fission An atom's nucleus can be split apart.

When this is done, a tremendous amount of energy is released in the form of both heat and light. The word fission means to split apart. Inside a nuclear reactor, such as OPAL, uranium atoms are split apart in a controlled chain reaction. In a chain reaction, neutrons released by the splitting of the atom go off and strike other uranium atoms, splitting those and releasing another lot of neutrons to continue striking more uranium atoms. In nuclear reactors, design features and control rods are used to regulate the splitting so it does not go too fast. If the reaction is not controlled and the fuel is almost pure uranium-235 or plutonium, you could have an atomic bomb. These conditions are not present in a nuclear reactor. Nuclear fusion Fusion is when smaller nuclei join together to make a large nucleus. The sun is powered by nuclear fusion of hydrogen atoms into helium atoms. This gives off heat, light and other radiation. Scientists have been working on controlling nuclear fusion for a long time, trying to make a fusion reactor to produce electricity. To date, prototype fusion reactors have consumed more energy than they have produced, but scientists believe that the future of fusion research is promising.

Question: A particle of alpha radiation is a nucleus of an atom of which element?
Answer: Helium